The invention relates to a process for dressing an internal or external gear tool, in particular a honing tool, mounted in a machine for fine machining of the tooth profiles of gear workpieces, with a dressing wheel which is engaged with the tool to be dressed in place of a gear workpiece and is set in rotation.
In conventional fine machining, known among other things also as tooth honing, of the tooth profiles of specially hardened workpieces in the form of gears with a tool whose flanks have an abrasive surface, that is, no uniformly oriented cutting edges, the tool must be dressed at specific time intervals. For this purpose the tool is generated with a dressing wheel whose tooth profiles are provided with a coating of granular diamond or other hard material. It is customary to leave the tool in the fine finishing machine and to replace the workpiece to be machined with a dressing wheel possessing the identical basic gear data. However, even if the greatest possible care is taken in dressing, the result and accordingly the quality of the gear workpieces subsequently machined with the dressed tool often are not satisfactory. The conventional generic process (DE 195 00 801 C2) provides for use of a dressing gear with a number of teeth greater than those of the gear workpiece. The modulus and helix angle are, however, identical to those of the gear workpiece, and the axis intersection angle (i.e. cross axis angle) in the dressing of the tool corresponds more or less to that employed in fine machining of the gear workpiece.
In machining of gear workpieces with a so-called disruptive profile, such as a shoulder projecting radially, limits are naturally imposed on the axis intersection angle. If this limit of the axis intersection angle is exceeded, the tool is engaged with the radially projecting area or areas of the gear workpiece and damages it or is itself damaged. Because of the disruptive profile of the gear workpiece, the permissible axis intersection angle in fine machining may be very small in these instances. But the cutting relationships deteriorate increasingly with decrease in the axis intersection angle, since the axial component of relative movement of the tool and gear workpiece becomes increasingly smaller. Since the helix angle of the dressing wheel is the same as that of the gear workpiece, the same helix angle is used in dressing of the tool as in machining of the gear workpiece, so that unfavorable cutting conditions also prevail. The quality of the dressed tool and accordingly that of the gear workpieces subsequently machined with it are as a result often unsatisfactory.
The object of the invention is to improve the process indicated above so as to make it possible to achieve more favorable cutting conditions during dressing of the tool.
The achievement of more favorable cutting conditions during dressing is attained by use of a dressing wheel with a helix angle differing from that of the gear workpiece, an axis intersection angle being set for the dressing wheel which differs from that for fine machining of the gear workpieces, in order to obtain the assigned helix angle of the tool.
The invention makes use of the fact that in order to achieve the desired helix angle it is not necessary for the helix angle of the dressing wheel to coincide with that of the gear workpiece and for the axis intersection angle to be the same as in fine machining of the gear workpiece. Since the dressing wheel and the tool have no disruptive profile, no limit is imposed on the axis intersection angle during dressing. Consequently, the tool may be dressed with an axis intersection angle which is more favorable, that is, a larger axis intersection angle with respect to the cutting conditions than in fine machining of a gear workpiece. This results in a dressed tool, and accordingly gear workpieces machined with it, of better quality. With a larger axis intersection angle the axial velocity component on the rolling circles increases, something which generally enhances transfer of the flank geometry of the dressing wheel to the tool and results in more uniform results and in longer tool life. The forces arising during dressing are reduced in the rolling circle area in particular. Hence the life of the dressing wheel is increased. The improved dressing result achieved under the more favorable cutting conditions is transferred to the gear workpiece during machining. Generally, then, more gear workpieces can be machined before dressing of the tool becomes necessary.
As was stated in the foregoing, the invention makes it possible to improve the cutting conditions in dressing of a tool in comparison to those prevailing during machining of the gear workpiece. This improvement in cutting conditions may be based not only on increase in the axis intersection angle but also on modification of the helix angle of the dressing wheel. For example, a tool used for fine machining of spur-toothed gear workpieces may be dressed by suitable selection of the axis intersection angle in dressing with a helical dressing wheel. A helical dressing wheel is more favorable than a spur-toothed dressing wheel from the viewpoint of machining conditions.
The axis intersection angle required for achievement of an assigned tool helix angle may be calculated on the basis of the helix angle of the dressing wheel and set manually. By preference, however, the axis intersection angle necessary for dressing is calculated by the machine control unit on the basis of the toothing data of tool and dressing wheel and is assigned by the unit.